Titration of an Antacid
Purpose:
In this lab you will determine the acid neutralizing power of a commercially available
antacid using a technique called back-titration.
Overview:
Hydrochloric acid is the acid secreted by the parietal cells in the stomach. The flow of
this acid increases when food enters the stomach. If you eat too much your stomach may
produce too much of the acid. This produces the condition called “heartburn” or
indigestion. To relieve this condition, antacids can be swallowed to neutralize the excess
acid. The key ingredient in any antacid is a base. This base reacts with the hydrochloric
acid to neutralize it. The table below contains a list of some antacids and the bases that
they contain.
Brand
Pepto-Bismol
Base
BiO(HOC6H4COO)
Milk of Magnesia
Mg(OH)2
Rolaids
NaAl(OH)2CO3 or CaCO3
Tums
CaCO3
Alka-Seltzer II
NaHCO3 and KHCO3
Maalox
Mg(OH)2 and Al(OH)3
Gaviscon
Al(OH)3
Acids are neutralized by these bases as illustrated below:
BiO(HOC6H4COO) + 3H+(aq) → Bi3+(aq) + H2O(l) + HOC6H4COOH(s)
Mg(OH)2(s) + 2H+(aq) → Mg2+ (aq) + 2H2O(l)
Al(OH)3(s) + H+(aq) → Al(OH)2+ (aq) + H2O(l)
Al(OH)2+ (aq) + H+(aq) → Al(OH)2+ (aq) + H2O(l)
CaCO3(s) + H+(aq) → Ca2+(aq) + HCO3- (aq)
HCO3- (aq) + H+(aq) → CO2(g) + H2O(l)
In this lab you will find the neutralizing capacity for a commercial antacid. To do this,
you will first dissolve an antacid in a measured excess of hydrochloric acid. This acid
will react with the antacid, but leave some unreacted hydrochloric acid. You will then
measure the excess hydrochloric acid using titration with sodium hydroxide. This
technique is a type of titration called back-titration.
Safety Precautions:
Safety goggles and aprons must be worn in lab at all times. Sodium hydroxide is a strong
base. Hydrochloric acid is a strong acid. Both can cause severe burns; wash all
contaminated areas thoroughly with cold water. Acids and bases are corrosive chemicals
and can cause burns to skin and eyes. Avoid contact and wash any contaminated area
thoroughly with cold water. Report any spills.
Procedure:
Preparation and Standardization of the NaOH Solution
1)
Mass approximately 0.6 g of KHP and record the mass to the nearest
milligram. In a 125mL Erlenmeyer flask, dissolve the KHP in 50-75mL of
deionized water and add 2-3 drops of phenolphthalein indicator.
2)
Prepare an approximately 0.2M solution of sodium hydroxide using a
volumetric flask. Add a small quantity of deionized water to a 500mL or
1000mL volumetric flask and add the appropriate amount of sodium
hydroxide beads. Add more deionized water to the line and mix.
3)
Rinse a buret with a few milliliters of the NaOH solution to remove any
impurities. Using a funnel, fill the buret with about 50mL of base. Open the
stopcock and allow a few mL to drain through the tip into a beaker to flush out
any trapped air bubbles. When the solution has settled in the buret, record the
volume to ±0.01mL reading the level from the top down. (Remember
significant figures: record numbers from the markings and a last number
estimated between the final two markings.)
4)
Add NaOH from the buret slowly to the KHP solution while swirling the
flask. Momentary flashes of pink color will occur and remain visible for
longer periods of time as the endpoint nears. The titration is complete when a
very FAINT PINK color persists throughout the solution for more than 30
seconds. Record the final volume of NaOH. (If the solution is dark pink, you
have overshot the endpoint by one or two drops of NaOH.) Do several
titrations so you have two to three very close determinations for the NaOH
concentration.
Titration of Antacid
1)
Carefully pipet two 50 mL samples of the 0.5M HCl solution into two clean
250-mL Erlenmeyer flasks, labeled 1 and 2.
2)
Obtain and mass two antacid tablets from the same brand. Add one tablet to
each flask containing HCl.
3)
Cover each flask with a watch glass and bring to a gentle boil for 5 minutes on
a hot plate. This helps the tablet to dissolve and expels any CO2 produced by
the reaction with HCl. Set it aside to cool. Some solid materials, used to bind
the tablet together may remain.
4)
Add 2-3 drops of phenolphthalein indicator, to the cooled solution of antacid
tablet containing the unreacted acid. The solution should be clear. If it is not
clear you may have consumed all of the acid with the antacid.
5)
Now take the first flask, slowly add NaOH from the buret while swirling the
flask. As you are adding NaOH, the solution in the beaker will change from
clear to pink. The endpoint is reached when the pink color persists for 15
seconds or more. As it gets more difficult to get rid of the pinkish color when
you swirl the flask, add smaller amounts. Read and record the final volume.
6)
Refill the buret proceed in the same manner to titrate the HCl in the second
flask.
Calculations
1) Calculate the concentration of NaOH for each titration with KHP. Find the
average concentration for the closest titrations.
2) Calculate the moles HCl that are neutralized from by each gram of antacid.
3) If stomach acid is typically 0.15M, what volume of stomach acid can be
neutralized with each tablet of antacid?